Travelling wave tubes



June 16, 1959 s. a. coHN 2,891,190

i TRAVELLING WAVE TUBES Filed April 29, 1954 '2 sheets-smet 1v INVENTOR Q ATTORNEY 'i J'n'e" f6, T95@ s. B. COHN 2,891,190

Y I l TRAVELLING WAVE TUBES Filed April 29, 1954 2 Shets-Sheet 2 INVENTOR l bwwew Unite States Patent Utilice 2,891,190 Patented June 16, 1959 TRAVELLING wAvE TUBES Seymour B. Cohn, Palo Alto, Calif., assigner to Sperry Rand Corporation, a corporation of Delaware Application April 29, 1954, Serial No. 426,362

Claims. (Cl. S15- 3.5)

This invention relates to travelling wave tubes, and more particularly, is concerned with means for providing a matched coupling between a coaxial transmission line and a wire helix transmission line.

One of the desirable properties of travelling wave tubes is that they are capable of amplifying ultra-high frequency signals over a `broad band of frequencies. This property of the travelling wave tube has not been fully exploited, however, because of the ditiiculty in matching the input and output transmission lines to the helix over the wide frequency band in which the travelling wave tube is inherently capable of operating. To take advantage of the broad `band properties of the traveling wave tube, coaxial line sections are generally used for the ultra-high frequency input and output circuits. Because the mode of transmission along the helix is different than the mode of transmission along the coaxial line and because the characteristic impedance of the helix varies as a function of frequency, the usual matching techniques, such as the use of matching transformers involving lumped impedances or other types of resonant matching, are inadequate.

In one method known to the prior art'of matching the helix to the input and output coaxial transmission lines, matching is achieved in two steps, namely, by smoothly converting the wave on the helix from the helix mode to a TEM mode and then matching the TEM-operating helix to the TEM-operating coaxial line. This mode conversion is accomplished in the following manner: An Outer conductor in the form `of a metallic shell surrounds the helix. The ends of the shell are tapered to provide very close spacing Ibetween the helix and the outer conductor at the ends of the helix, where the helix joins the input and output coaxial lines.

The theory is that in the middle of the helix Where the shell is remote, the electric eld vectors `extend axially between adjacent turns of the helix in aconventional manner typical of the helix mode of propagation. At the ends of the helix, where the spacing between the outer conductor and the helix is reduced, the electric `field lines extend between the helix and the outer conductor rather .t

than lbetween adjacent turns, and so propagation canzbe converted to the TEM mode, which mode can then be matched to the TEM mode of propagationin the coaxial input and output transmission lines. This second matching step requires that the characteristic impedance of the TEM-operating helix be the same as the characteristic impedance of the associated coaxial line.

The characteristic impedance of the TEM-operating helix depends on the spacing between the helix and the outer conductive shell. To get a characteristic impedance of the order of fifty ohms (the impedance of standard coaxial line) requires extremely close spacing, of the order of a tenth of the diameterof the wire of the helix. Since such close spacing is difficult to achieve in practice, one solution heretofore proposed is, that `the coaxial lines be tapered to `provide a higher characteristic impedance where they join to the helix, thus permitting wider spacing `between the helixand `the outer conductive shell. Howh elix from the outer conductive shell, propagation in the TEM mode is only partially achieved so that the matching characteristics desired are not in fact achieved in practice.

It is the general object of this invention to avoid and overcome the foregoing and other diiculties in and objections to the prior art practices by the provision of an improved high frequency travelling wave tube for amplifying a wide |band of frequencies.

Another object of this invent-ion is the provision of wide band coupling means between a wire helix transmission line and a 50-ohm coaxial transmission line.

These and other objects of the invention which will become apparent as the description proceeds are achieved by providing in a travelling wave tube having a coaxial input line section and a wire helix, broad band coupling means between the coaxial line and the helix comprising a conductive circular horn having fa throat portion of inner diameter slightly larger than the diameter of the wire helix, with its axis of revolution being aligned with the longitudinal axis of the helix. The wire helix terminates in the region of the mouth of the horn. and the coaxial line section terminates with its outer conductor joined `at the throat portion of the horn and with its inner conductor extending into the throat portion, the longitudinal axis of the coaxial line section being perpendicular to the axis of revolution of the horn. A helical conductor comprising a` metal strip that is Hat in cross-section and tapered in width connects the end of the wire helix to the inner conductor of the coaxial line. To provide a match between the helix wire and strip, the pitch angle of the Wire helix and the helical strip conductor are equal at their point of junction, the width of the strip at the junction being such as to have the equivalent capacitance of the round wire at that point. The width of the strip and spacing of the horn in the throat reg-ion thereof, Where the spiral conductor connects to the inner conductor of the coaxial line, are such as to provide a strip line formed by the spiral conductor and the horn that has the same characteristic impedance as the coaxial line section.

`For a better understanding of the invention, reference should be .had to the accompanying drawing, wherein:

Figl is an elevational view, partly in section, of a travelling wave tube incorporating the features of the present invention;

Fig. 2 is a cross-sectional View taken substantially on the line 2-2 of Fig. l; and

Fig. 3 is a cross-sectional view taken substantially on the line 3-3 of Fig. l.

`Referring to Fig. l, there is shown a travelling wave tube for use as an amplifier for ultra-high frequencies. The tube includes an evacuated envelope including an elongated metal shell 1t). For ease of assembly the shell 1,0 is divided into two parts which are joined by means of a collar 12, the collar being brazed or otherwise secured in sealed relationship to the adjacent portions of the shell 10.

, Secured in sealed relationship at the ends of the shell 10 are end plates 14 and 16 to which are mounted respectively an electron gun assembly, indicated generally at 18 and a collector electrode assembly, indicated generally at 20. For ease of assembly, the end plate 16 includes a demountable inner portion 17 secured by screws 19. The electron gun assembly includes a mounting collar 22to which is secured a base (not shown) as by means of a glass seal indicated at 24. An electron gun 26, which is supported from the base, may fbe of any conventional design capable of directing a stream of electrons along an axis, initial focusing of the electron *beam being provided by a suitable focusing electrode 2:8.

The collector electrode 20 may be of any Well known design for absorbing the energy of the electron stream. The collector is preferably provided with cooling ns 30;

a glass seal 32 isolates the collector electrode electrically from the shell of the tube. For the purpose of ease in assembly, a brazed joint is provided :between the glass seal 32 and the end plate 16 by means of collar elements 34 and 36. Extending along the longitudinal axis of the cylindrical shell 10 is a wire helix 38 which is supported along its length by ceramic rods 40. The rods are preferably of an attenuating type described in the co-pending application Serial No. 427,439, iiled May 4, 1954. Discs 46 of ceramic material in turn position the rods in proper spaced relationship with respect to the cylindrical shell 10, the discs 46 being positioned at intervals along the length of the rods to provide a rigid support for the helix 38 (see Fig. 3). The ultra-high frequency input to the travelling wave tube is provided by a coaxial line, the input section of which is indicated generally at 48. The output from the tube is in turn provided by a second coaxial line section, indicated generally at 50. Matched coupling between the input and'output coaxial line sections and the helix 38 of the tube is achieved in accordance with the unique features of the present invention in the'following manner.

Positioned at the input end of the cylindrical shell 10 is a circular horn 51 having its axis of revolution aligned with the longitudinal axis of the helix 38. The horn 51 includes a Haring portion 52 and a throat portion 54, the flaring portion of the horn l terminating against the inner Wall of the cylindrical shell 10, while the throat portion 54 terminates against the end plate 14. The plate 14 has an aperture 56 for passage of the electron stream. A similar horn 57 :at the other end of the tube includes a flaring portion 58 terminating against the inner wall of the cylindrical shell 10, and a throat portion 60 terminating against the removable portion 17 of the end plate 16. An opening 62 in the end plate is aligned with the throat portion of the horn 57 to permit passage of the electron stream therethrough.

The input coaxial line section 48 extends into the interior of the tube through an opening in the cylindrical shell 10, the outer conductor 70 of the coaxial line and the cylindrical shell being joined in sealed relationship where the coaxial line passes into the interior of the tube so as to maintain the vacuum seal. The outer conductor 70 of the input coaxial line 48 terminates in an opening in the throat portion 54 of the horn 51. The inner conductor 72 of the input coaxial line 48 extends into the throat portion of the horn 51. A glass bead supports the inner conductor 72 and at the same time provides a vacuum tight seal in the coaxial line 48 (see Fig. 2).

Extending between theend of the helix 38 (the helix 38 terminating in the region of the mouth of the horn 4 Where R is equal to W divided by 2S. Suitable values for W `and S are .5 and .086 respectively.

51) and t-he end of the inner conductor 72 which projects into the mouth of the horn 51 is a fiat band helical conductor 76. This conductor is made from a metal strip that is flat in cross-section and tapered in width, Ibeing narrow where it joins to the helix 38 and being considerably wider where it joints to the Vinnerconductor 72. At the point where it joins the inner conductor 72 of the coaxial line 48, the strip conductor 76, because of its close spacing to the throat portion 54 of the horn 51, acts as a strip-above-a-ground-plane type of transmission line propagating energy in the TEM mode. To achieve a match between this transmission line and the coaxial line 48, this strip line must have a characteristic impedance equal to that of the coaxial line 48, namely, a characteristic impedance of the order of 50 ohms. The spacing S between the strip conductor 76 and the throat portion 54, and the width W of the strip conductor 76 may be determined from the following formula:

Any discontinuity that exists at the junction between the strip conductor 76 and the coaxial line 48 may be substantially eliminated by the extension of the strip conductor beyond the inner conductor 72, as indicated at 75 in Fig. 2. By trial and error, the end 75 of the strip conductor can be shortened until a good match is achieved.

The narrow end of the strip conductor 76 is designed to provide a smooth transition between the helix 38 and the strip conductor 76. For this reason where the end of the strip conductor is joined to the helix, the width of the strip conductor 76 is chosen so as to have the same capacitance per unit length as the wire of the helix relative to a remote ground plane. It can be shown that a thin flat strip having a width twice the 'diameter of a round Wire has the same equivalent capacitance. For example, if the helix wire is .080 in diameter, the strip conductor 76 narrows down to a width of .160 where it joins to the helix. Also to provide a smooth transition 4between the strip conductor 76 and the helix 38, the strip conductor should be wound so that the pitch angle matches the pitch angle of the wire helix where the two are joined. Smooth transition between the helix mode of propagation and the TEM mode of propagation along the 'strip conductor 76 is achieved by the flaring portion 52 of the horn 51.

Similarly the output coaxial line 50 is coupled to the vhelix 38 by means of a helically Wound strip conductor 78. r[The output coaxial line 50` includes an outer conductor 80 which is joined to the throat portion 60 of the horn 57, and an inner conductor 82 which is joined to vthe strip conductor 78 through an opening 84 in the throat portion 60. Although the inner and outer conductors of the output coaxial line 50 are shown as being considerably larger in diameter than that of the input line 48, the ratio of diameters is actually kept the same so that the characteristic impedance of the output coaxial line is likewise substantially 50 ohms, the larger diameters being provided to accommodate the larger power at the output end of the tube.

From the above description it will be recognized that the various objects of the invention have been achieved by the provision of an improved travelling wave tube. By using a helical strip conductor and tapered horn, transition from a helix mode to TEM mode type of propagation is achieved. Furthermore the strip conductor and horn combine to form a transmission line of very low characteristic impedance to provide a direct match to the low impedance coaxial output and input lines. Yet this low characteristic impedance is achieved without severe physical limitations such as extremely close spacing between the horn and the helical conductor.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A travelling wave tube comprising a helix of conductive wire, means for projecting a stream of electrons along the longitudinal axis of the wire helix, an evacuated envelope enclosing the wire helix and said means, the envelope including a hollow cylindrical shell surrounding the wire helix, a pair of circular horns positioned at opposite ends of the wire helix within the shell with their axes of revolution aligned with the longitudinal axis of the wire helix, each horn having an elongated cylindrical throat portion of inner diameter slightly larger than the diameter of the wire helix and a mouth portion Haring out from an inner diameter substantially the same as that of said throat portion to a larger inner diameter corresponding substantially to the inner diameter of the shell, the

i'nouth portions of the horns being directed toward each other and positioned longitudinally adjacent Vthe respective ends of the Wire helix, input fand output coaxial line sections of relatively low characteristic impedance extending through the shell at right angles to thelongitudinal axis of the wire helix, the coaxial line sections terminating in T-junctions with the `respective horns at the throat portions of the horns, with the inner conductors of the coaxial line sections projecting into `the respective throats of the associated horns, and helical conductors each cornprising a flat metallic strip of width tapered from a wide end to a narrow end and Wound in a helix of substantially the same diameter as the wire helix, the helical conductors electrically connecting the inner conductor of each of the coaxial line sections to the respective ends of the wire helix, each helical conductor being joined to the wire .helix at the narrow end of the strip and being joined to `the inner conductor of the respective coaxial line section adjacent the wide end of the strip, the pitch of the helical conductors being equal to the pitch of the wire helix at their point of junction, and the spacing of the helical conductor from the horn and width of the strip at the wide end being such as to provide with the horn a strip transmission line having the same characteristic impedance as the coaxial line sections.

2. A travelling wave tube comprising a helix of conductive wire, means for projecting a stream of electrons along the longitudinal axis of the wire helix, an evacuated envelope enclosing the Wire helix and said means, a pair of circular horns positioned at opposite ends of the wire helix within the envelope with their axes of revolution aligned with the longitudinal axis of the wire helix, each horn having a throat portion and a mouth portion, the throat portion of each horn having an inner `diameter slightly larger than the outer diameter of said helix, one end of a mouth portion of each horn adjoining ya throat portion, said one end having substantially the same inner diameter as that of the throat portion, each mouth portion having an inner surface which flares outwardly of said axes from said one end tothe other end of the mouth portion to thereby provide a tapered horn section, the mouth portions of the horns being directed toward each other and positioned longitudinally adjacent the respective ends of the wire helix, input and output coaxial line sections terminating in T-junctions with the respective horns at the throat portions of the horns, with the inner conductors of the coaxial line sections projecting in directions transverse said axis into the respective throats `of the associated horns, and helical `crmdnctors each comprising a flat metallic strip of width tapered from a wide end to a narrow end and wound in a helix, the helical conductors electrically connecting the inner conductor of each of the coaxial line sections to the respective ends of the Wire helix, each helical conductor being joined to the wire helix at the narrow end of the strip and being joined to the inner conductor of the respective coaxial line section adjacent the wide end of the strip, the spacing of the helical conductor from the horn and width of the strip at the Wide end being such as to provide with the horn a strip transmission line having the same characteristic impedance as the coaxial line sections.

3. In a travelling wave tube having a coaxial input line section and a wire helix, means for providing a broad band matched coupling between the coaxial line section and the helix comprising a conductive circular horn having a throat portion of inner diameter slightly larger than the diameter of the wire helix, the axis of revolution of the horn being Ialigned with the longitudinal axis of the wire helix, said horn having a mouth portion with one end thereof adjoining said throat portion, said one end having substantially the same inner diameter as that of said throat portion, said mouth portion having an inner surface which ares outwardly of said axis from said one end to the other end of the mouth portion to thereby provide a tapered horn section, the wire helix terminating in the region of the mouth of the horn, the coaxial input line section extending perpendicularly to the axis of` the horn and having the outer conductor thereof joined at the throat portion of the horn, the inner 'conductor of the coaxial line section extending into the throat portion of the horn, and a helical conductor comprising a strip that is at in cross-section and tapered in Width from a wide end to a narrow end, the diameter of the helical conductor being substantially equal to the diameter of the wire helix, the helical conductor being joined to the end of the wire helix at the narrow end of the strip, the Vpitch angle of the wire helix and Vthe helical conductor being equal at the point of junction, the innerconductor of the coaxial line being joined to the helical conductor adjacent the wide end of the strip, the axial length of the helical conductor being of several times the diameter of the helical conductor, and the width of the strip and spacing from the horn in the throat region thereof being such as to provide a strip line having the same characteristic impedance as the coaxial line `section whereby a gradual transition between the coaxial line and vthe wire helix is provided.

4. In a travelling wave tube having a coaxial input line section and a wire helix, means for providing a broad band matched coupling between the coaxial line section and the helix comprising a conductive horn having a throat portion and a mouth portion, the axis of the horn being aligned with the longitudinal axis of the Wire helix, said throat portion having a slightly larger inner diameter than the outer diameter of said helix, said mouth portion having one end adjoining said throat portion, said one end having substantially the same inner `diameter `as that of -said throat portion, said mouth portion having an inner surface which flares outwardly of said axis from said one end to the other end of the mouth portion to thereby provide a tapered horn section, the wire helix terminating in the region of the mouth of the horn, the coaxial input line section extending perpendicularly to the axis of the horn and having the outer conductor thereof joined at the throat portion of the horn, the inner conductor of the coaxial line section extending into the throat portion of the horn, and a helical conductor comprising a strip that is flat in cross-section and tapered in Width from a wide end to a narrow end, the diameter of the helical conductor being substantially equal to the diameter of the wire helix, the helical conductor being joined to the end of the wire helix at the narrow end of the strip, the pitch angle of the wire helix and the helical conductor being equal at the point of junction, the inner conductor of the coaxial line being joined to the helical conductor adjacent the Wide end of the strip, the axial length or the helical conductor being of several times the diameter of the helical conductor, and the width of the strip and spacing from the horn in the throat region thereof being such `as to provide a strip line having the same characteristic impedance as the coaxial line section whereby a gradual transition between the coaxial line and the wire helix is provided.

5. In a travelling wave tube having a coaxial input line section and a wire helix, means for providing a broad yband matched coupling between the coaxial line section and the helix comprising a conductive horn having a throat portion and a mouth portion flaring outwardly from the axis of said horn `from an inner diameter substantially equal to that of said throat portion in the vicinity of said throat portion to an appreciably larger inner diameter remote from said throat portion for providing a tapered horn section, the axis of the horn being aligned with the longitudinal axis of the wire helix, the wire helix terminating in the region of the mouth of the horn, the coaxial input line section extending perpendicularly to the :axis of the horn and having the outer conductor thereof joined at the throat portion of the horn, the inner con- -ductor of the coaxial line section extending into the throat portion of the horn, and a helical conductor comprising a strip tapered from a wide end to a narrow end, the

helical conductor being joined to the end of the wire helix at the narrow end of the strip, the inner conductor of the coaxial line being joined to the helical conductor adjacent the wide end of the strip, and the width of the strip and spacing from the horn in the throat region thereof being such as to provide a strip line having the same characteristic impedance as the coaxial line section whereby a gradual transition between the coaxial line and the wire helix is provided.

' 6. Apparatus as defined in claim 5 wherein the strip forming the helical conductor extends at the wide end thereof beyond the junction point with the inner conductor of the coaxial line, the extended portion providing a matched junction between the 'coaxial line and said strip line.

7. Means for coupling a helix transmission line to a coaxial line, said means including a helically wound metallic strip, the strip being tapered in width from a wide end to a narrow end with the narrow end of the strip being connected to the end of the helix transmission line, the spacing between adjacent turns of the strip at the wide end of the strip being smaller than the width of the strip at the wide end of the strip, the longitudinal axis of the helically wound strip being substantially aligned with the longitudinal axis of the helix transmission line, and flared conductive horn means having an inner surface of tapered diameter concentrically surrounding the helically wound strip, said horn means being more closely spaced from the strip at the wide end of the strip than at the narrow end of the strip, the strip adjacent the wide end thereof and the adjacent portion of said conductive means being joined respectively to the inner conductor and outer conductor of said coaxial line, the coaxial line extending at right angles to the longitudinal axis of the helically wound strip.

8. Means for coupling a helix transmission line comprised of a Wire conductor to a coaxial line, said means including a helically wound metallic strip connected to the end of the helix transmission line, the longitudinal axis of the helically wound strip being substantially aligned with the longitudinal axis of the helix transmission line, the end of said strip remote from said helix transmission line having a substantially larger width than the ydiameters of said wire conductor and the inner conductor of said coaxial line conductor and flared conductive horn means having an inner surface of tapered diameter concentrically surrounding the helically wound strip, said means being more closely spaced from the strip at the end remote from the helix transmission line, the strip adjacent said remote end and the adjacent portion of said conductive means being joined respectively to the inner conductor and outer conductor of said coaxial line, the coaxial line extending at right angles to the longitudinal axis of the helically wound strip. f

9. A travelling wave tube including a helical conductor, means for projecting a stream of electrons along the longitudinal axis of the helical conductor, a conductive horn having a throat portion and a flared mouth portion having an increasingly larger inner diameter in a direction away from lsaid throat portion, the axis of the horn lbeing aligned with the longitudinal axis of the helical conductor, and a coaxial line section extending in a direction transverse said axis and having the outer conductor thereof connected to the throat portion of the horn and having the inner conductor thereof joined to the helical conductor at =a point within the throat portion of the horn, the portion of the helical conductor within the region of the horn comprising a conductive iiat strip of increasing width and wound with an increasing pitch from the point where the helical conductor enters the horn to the point where the helical conductor joins the inner conductor of the coaxial line.

10. A travelling wave tube including a helical conductor, means for projecting a stream of electrons along the longitudinal axis of the helical conductor, a flared conductive horn means having lan inner surface of tapered diameter rcoaxially positioned and lsurrounding a portion of the helical conductor adjacent one end thereof, and a coaxial line extending in a direction transverse said axis and having the inner conductor and outer conductor connected respectively to the helical conductor and adjacent portion of said conductive means, the portion of the helical conductor within the region surrounded by said conductive horn means comprising a conductive flat strip of increasing width and wound with an increasing pitch in progressing toward the point where the helical conductor joins the inner conductor of the coaxial line.

References Cited in the le of this patent UNITED STATES PATENTS 2,578,434 Lindenblad Dec. 11, 1951 2,615,141 Hansell Oct. 21, 1952 2,637,775 Lund May 5, 1953 2,672,571 Harman 1 Mar. 16, 1954 2,727,179 Lally Dec. 13, 1955 2,774,006 Field et al Dec. 11, 1956 2,788,465 Bryant et al. Apr. 9, 1957 

